In the pulping industry, star-type rotary feeders are often used to convey material, for example, wood chips (or other types of comminuted cellulosic fibrous material), from one vessel to another, or from one set of process conditions to another. For example, such feeders are typically used to transfer chips from one pressurized state to another, typically, from a lower to a higher pressurized state. Conventional star-type feeders, for example, those sold by Ahlstrom Machinery of Glens Falls, N.Y. and serviced by Ahlstrom Services, of Pell City, Alabama, typically comprise pocketed rotors mounted for rotation within a cylindrical housing. The housing typically contains two or more openings or ports which accept chips under one set of process conditions, for example, pressure or temperature, and discharge chips under a different set of process conditions. The rotors typically comprise two or more pockets or cavities which communicate with the openings in the housing to accept chips introduced at one opening and are then transferred by rotation to the housing discharge opening. Typical star-type feeders include those sold under the names Airlock Feeder, Chip Meter, Low Pressure Feeder, or High Pressure Feeder, among other types of devices.
Since these star-type feeders are designed to transfer material from one set of process conditions to another, they are also designed to isolate the process conditions, that is, prevent leakage of liquids and gasses, between one state and another state. For this reason, the clearance between the outside diameter of the rotor and the inside diameter of the housing is typically tightly toleranced, and closely monitored and controlled to ensure that as little leakage as tolerable by the process occurs.
However, the material being transferred, for example, hardwood or softwood chips, in a dry, steamed or slurried state, makes it difficult to maintain the tight clearances without making some accommodation for the effect the rigid chips, or tramp material (such as stones, sand, nuts and bolts) can have upon the surfaces that define the clearance between the rotor and the housing. The surfaces of both the rotor and the housing, especially the leading edges exposed to the openings (especially the inlet) in the housing, can become worn or damaged during operation. This damage can increase the clearance between the running surfaces and result in increased leakage of process fluids. Damaged surfaces or debris can also increase the friction between the running surfaces and result in increased electrical loads on the motor or drive train turning the rotor.
Conventionally, the leading internal edge of the housing that confronts the material as it is transferred by the rotation of the rotor is critical to preventing the lodging of material between the inner surface of the stationary housing and the outer surface of the rotating rotor. This edge is typically designed and then monitored to minimize the possibility of material entering the clearance between the rotor and housing. The edge is typically given a sharp contour that acts to shear any outer material that is carried by the pocket of the rotor into the housing, and is commonly referred to as a "shear edge". The "shear edge" is typically a machined edge to ensure a sharp, clean contour. The feeder housing is typically cast, for example, cast in stainless steel, and this shear edge is machined at the same time the internal surface of the housing is machined to provide the desired clearance between the housing and the rotor. In order to further protect this shear edge, star-type feeders, for example, a Low Pressure Feeder, may also include a protective baffle positioned above the shear edge to prevent large particles from impacting and damaging the shear edge. This baffle, typically referred to as a "doctor blade" (though it does not act as a conventional doctor blade, for example, as used to doctor pulp from a rotating drum cylinder) is typically welded separately to the housing inlet.
Conventionally, star-type feeders have limited service life before the external surface of the rotor or the internal surface of the housing becomes sufficiently damaged that one or both surfaces need to be replaced, repaired, or "re-built". For example, at Ahlstrom Services, a "rebuild" typically comprises "overlaying" either damaged surface with weld material and then machining the overlaid weld material to the desired dimensions. Due to the exposure of the shear edge and its potential to be damaged during operation, the shear edge of the housing is typically more prone to damage. It is frequently damage to this shear edge, and sometimes this shear edge alone, that necessitates a rebuild of a feeder. For example, in one pulp mill the life of a Low Pressure feeder was limited to only 6 to 8 months due to excessive damage to its shear edge.
According to the invention it has been recognized that since the shear edge of a feeder is prone to such accidental damage, that providing a feeder with a shear edge that is readily replaceable can extend the service life of a feeder so that fewer rebuilds are necessary. In addition a more easily replaceable shear edge can provide less "down time" by the pulp mill in order to service the feeder.
According to one aspect of the invention a star-type feeder for transferring comminuted fibrous material is provided having as components: a housing, having at least one inlet opening for receiving material and at least one opening for discharging, and an internal shear edge adjacent the inlet; a rotor, rotatably mounted with the housing and having pockets for accepting material introduced to said housing; and a power source which rotates the rotor in said housing; and wherein the housing shear edge is readily replaceable.
According to one aspect of the present invention, a method of treating comminuted cellulosic fibrous material utilizing a star feeder having a rotor with a plurality of pockets rotatable within a cylindrical housing with an inlet and an outlet, a clearance between the housing and the rotor, and a shear edge in the inlet at a downstream portion of the inlet in the direction of rotation of the pocketed rotor is provided. The method comprises the steps of: (a) Feeding comminuted cellulosic fibrous material into the inlet. (b) Rotating the pocketed feeder to accept comminuted cellulosic fibrous material from the inlet and to carry the material past the shear edge to the outlet. (c) Discharging the comminuted cellulosic fibrous material from the pocketed rotor through the outlet. (d) When the shear edge is worn to approximately the point that excess leakage occurs or is substantially imminent, replacing the shear edge with a new shear edge while the practice of steps (a)-(c) is interrupted. And (e) repeating steps (a) through (d).
Where the shear edge is held in place by readily removable fasteners (such as screw threaded fasteners), step (d) may be practiced by removing the fasteners, removing the entire shear edge which was held on by the fasteners, replacing the shear edge, and holding the replaced shear edge in place with the fasteners. The shear edge may be provided by outer and inner shear plates which are attached by the fasteners in a stack, step (d) being further practiced by unfastening the fasteners, changing the positions of the outer shear plate and inner plate, and reattaching the fasteners. Where the fasteners are screw threaded and cooperate with internally threaded bores, step (d) is practiced by unscrewing the fasteners and then screwing the fasteners back into the threaded openings to reattach a new shear plate. Step (d) may also be practiced using a shear plate which has a shear edge with hardness properties of at least 10% (e.g. at least about 50%) greater than the hardness properties of the housing.
The method may be further practiced using a removable protective baffle mounted in the housing inlet in a position above the shear edge, providing protection for the shear edge. In that case there are the further steps of (f) readily removing the protective baffle prior to replacing the shear edge (e.g. by detaching and replacing the shear plate), and (g) after replacing the shear edge, reinstalling a protective baffle. The protective baffle may be held in place by screw threaded fasteners extending into and through the star feeder housing, in which case steps (f) and (g) are practiced by unscrewing the screw threaded fasteners holding the protective baffle in place, and re-screwing the fasteners back into place once the shear edge has been replaced; and positioning the protective baffle so that it engages at least one fastener for the shear plate.
Step (d) may alternatively be practiced by rotating the shear plate approximately 180.degree. about a horizontal axis so as to provide a new shear edge, and reinstalling the same shear plate with a new shear edge. Step (c) may be practiced in part by utilizing a conventional steam purge, and then exhausting the steam from the pocketed rotor after the steam is purged, also conventional per se.
Where the shear edge is provided by a distinct shear plate adjustably mounted with respect to the housing so that the position of the shear edge with respect to the pocketed rotor can be adjusted, step (d) may be practiced by adjusting the position of the shear edge so that it is closer to the pocketed rotor. For example, where the shear plate is adjustably mounted to the housing by a plurality of screw threaded fasteners received by internally screw threaded openings in the housing, and passing through a plurality of slots in the shear plate, the slots elongated in a substantially radial dimension, step (d) may be practiced by loosening the screw threaded fasteners, adjusting the position of the shear plate by sliding the shear plate so that the elongated openings move with respect to the fasteners with the shear plate guided by the fasteners in the elongated openings, and tightening the fasteners.
The invention also relates to a method of refurbishing the star feeder in a pulp mill, the star feeder having a cylindrical housing with inlet and outlet, a pocketed rotor disposed within the housing and having a clearance with respect thereto, and rotatable about an axis within the housing, and a shear edge formed in the inlet at the portion thereof closest to the rotor at the most downstream portion of the inlet in the direction of rotation. The method comprises the steps of: (a) Stopping rotation of the pocketed rotor. (b) Cutting out the shear edge to form a recess in the housing. (c) Forming fastener receiving openings in the housing adjacent the recess. (d) Installing a shear plate in the recess, the shear plate having a shear edge, fastening the shear plate to the housing with fasteners extending into the fastener receiving openings so that the shear edge of the shear plate is adjacent where the original shear edge of the housing was, and so that it functions to minimize entry of material into the clearance between the rotor and the housing, and to shear any large material that attempts to enter the clearance. And (e) restarting operation of the star feeder.
Step (d) may be practiced by placing first and second plates, in a stack, in the recess. There may be the further step (f) of when the shear edge on the shear plate becomes worn, loosening the fasteners, replacing the shear edge (such as by sliding the plate substantially radially with respect to the clearance/rotor), and tightening the fasteners. The inlet may include a protecting baffle for protecting the shear edge from large particles and tramp material, and there may be the further steps of: (g) removing the protective baffle; (h) forming a plurality of openings in the housing past the inlet adjacent where the protective baffle is removed; and (i) fastening a replaceable protective baffle to the housing using fasteners passing into the openings formed in step (h).
According to yet another aspect of the invention, a star feeder is provided comprising the following components: a generally cylindrical housing having an interior and an inlet and outlet cooperating with the interior. A pocketed rotor mounted in the interior and rotatable in a direction of rotation with respect to the housing so that each pocket thereof, during rotation, moves from a position in communication with the inlet to a position in communication with the outlet, in a direction of rotation thereof; the rotor and housing interior having a clearance therebetween. A shear edge disposed adjacent the clearance in the downstreammost portion of the inlet, in the direction of rotation. And the shear edge mounted so that it is readily replaceable.
The shear edge may comprise an edge of a plate, and the plate may be mounted to the housing by a plurality of fasteners so that the plate is readily movable or replaceable. The plate may include a plurality of elongated openings therein extending in a dimension toward the clearance and the fasteners engaged in the elongated openings, and upon loosening of the fasteners the plate is movable with respect to the fasteners, the elongated openings sliding with respect to the fasteners, and upon tightening of the fasteners the plate is secured in place with respect to the housing with the shear edge thereof properly positioned with respect to the clearance. A readily replaceable protective baffle may be disposed above the shear edge to protect the shear edge from large particles and tramp material, and the protective baffle may comprise a mounting portion having a plurality of through extending openings therein. The housing has a plurality of through extending openings cooperating with the openings in the protective baffle mounting portion, and the assembly may further comprise a plurality of fasteners passing through the mounting portion opening and the housing opening to releasably hold the protective baffle to the housing.
It is the primary object of the present invention to provide a star feeder, and a method of treating comminuted cellulosic fibrous material utilizing a star feeder, which maximizes the time between rebuilds, by providing a readily replaceable shear edge. This and other objects of the invention will become clear from a detailed description of the invention and from the appended claims.